a. Technical Field
Embodiments of the present invention relate to a drill string element such as a drill string stabiliser, and an associated bottom hole assembly, drill string, rig which may be for the drilling of an oil or gas well, method of retrofitting, computer program, and non-transitory computer readable medium.
b. Background Art
In the drilling of a wellbore for an oil or a gas well a drill bit, located at a remote end of a drill pipe (or ‘drill string’), is rotated to cut the wellbore through the ground.
Conventionally, rotation of the drill bit may be driven, in part, by a ‘mud motor’ which is located above the drill bit and which forms part of the drill string. The mud motor receives drilling fluid (commonly known as ‘mud’) which is delivered from the surface through the drill pipe. Movement of the drilling fluid is converted into rotational movement of the drill bit by a suitable mechanism—such as a helical flow path for the fluid through the mud motor which imparts a rotational force on a part of the mud motor which is coupled to the drill bit.
The drilling fluid is passed towards the drill bit through the drill string and leaves the drill string towards remote end thereof (normally at the drill bit). The drill fluid is, therefore, expelled into the wellbore. The fluid then travels up the wellbore, around the drill string, to the surface (as a result of more drilling fluid being pumped down the drill string at the surface to drive further rotation of the drill bit).
As is understood in the art, the drilling fluid also serves other purposes including lubrication and the transportation of cut material from the remote end of the wellbore to the surface.
The mud motor is, as will be appreciated, a form of ‘downhole motor’ in that the motor is part of the drill string which is located within the wellbore during use. Such motors include, for example, turbine motors (in which the fluid passes over a series of turbine blades to drive rotational movement) and positive displacement motors (in which a rotor and stator are cooperatively shaped such that the fluid pumped through the motor will cause rotation of the rotor with respect to the stator).
In conventional systems, the drill bit is rotated from the proximal end of the drill string (i.e. generally at or towards the ground surface)—such systems may or may not include a mud motor as well. Accordingly, the system may include one or more motors (which may be electrically powered motors) which are supported above or on the ground surface and which are mechanically coupled to the drill string. Such a system may be a top drive system in which the or each motor is suspended above the drill string and is mechanically coupled thereto. Such a system may, alternatively, be a rotary table based system in which the rotation of the drill string is driven by a rotary table which is located generally adjacent the wellbore substantially at the ground surface level (but potentially spaced apart from the ground surface by one or more other components of the system).
There is a need to keep the drill string in the desired position within the wellbore during the drilling operation so that the drill bit acts on the ground to cut the wellbore in the desired direction. Often this entails keeping the drill string centralised within the wellbore.
One or more stabilisers are provided as part of the drill string to space the drill string from a wall of the wellbore to keep the drill string in the desired position within the wellbore. These one or more stabilisers may also be used with rotary steerable tools in which a part of the drill string is spaced from the wall of the wellbore but the drill bit itself may be steerable towards the wall of the wellbore.
The stabilisers may take many different forms but typically comprise members which extend radially from the drill string towards the wellbore wall such that they contact the wellbore wall to restrict lateral movement of the drill string within the wellbore.
The drill string must be allowed axial movement, so that the drilling operation is not substantively impeded by the stabilisers, and the stabilisers must allow the flow of drilling fluid between the drill string and the wellbore wall to the surface.
Parts of the wellbore which do not have a casing (i.e. tubing which is typically cemented in place along the wellbore to separate the ground at the wellbore wall from the inside of the wellbore) are known as ‘open holes’.
The ground at the wellbore wall in an open hole is stabilised, in part, by the drilling fluid which passes between the wellbore wall and the drill string as it travels up the wellbore. This mud exerts a force against the wellbore wall to resist collapse of parts of the wellbore wall into the open hole of the wellbore. In addition, the fluid in the drilling fluid may, in places in which the ground is permeable for example, pass into the ground leaving behind an accumulation of ‘mud’ from the drilling fluid. This mud or filter cake forms a thin layer on the wellbore wall which aids in stabilisation of the wellbore.
Nevertheless, parts of the ground of the wellbore may partially collapse into the wellbore and may be carried by the drilling fluid up the wellbore along with cuttings excavated by the drill bit from the remote end of the wellbore.
To allow the passage of drilling fluid past, the stabilisers typically include elongate members which extend along a length of the drill string (as well as radially). Each such member is separated from its neighbouring member to allow the passage of drilling fluid between the members.
If there is an accumulation of debris adjacent the stabilisers, however, this can impede or even prevent axial movement of the drill string with respect to the wellbore. Such an accumulation may occur as a result of material from the ground collapsing into the wellbore and/or as a result of the cuttings from the action of the drill bit. The situation may be exasperated by the poor selection of an appropriate form of drilling fluid for the ground through which the wellbore is being drilled.
In some wellbores the ground surrounding part of the wellbore may swell (e.g. as a result of absorption of the drilling fluid). In addition, these and other materials in the ground may be pressed into the wellbore by the weight of the surrounding material—thus increasing the quantity of debris in the wellbore. Splintering of material from the wall of the wellbore may also occur as a result of over-pressured formations in the ground surrounding the wellbore, poor selection of drilling fluid, the absorption of drilling fluid by surrounding materials, and the like.
This is generally known a ‘packing off’ and commonly occurs in the region of the stabilisers (as the debris is packed between the drill string and the wellbore wall and inhibited from being removed by the stabilisers of the drill string).
The packing off of the drill string may cause the drill string to become stuck. This prevents removal of the drill string from the wellbore and/or continued drilling of the wellbore.
The drilling fluid, in such a situation, is often trapped and prevented from moving past one or more stabilisers by the debris. As such, any axial movement of the drill string (and hence the stabilisers which form a part of that drill string) requires the drilling fluid to be compressed or extended. This creates a hydraulic locking action which further inhibits axial movement of the drill string within the wellbore. In such events, rotation of the drill string may also be inhibited or substantially prevented—as a result of a number of different factors including increased friction at the drill bit and/or stabilisers (or elsewhere along the drill string) due to the delivery of drilling fluid being terminated (to prevent excessive fluid pressures) and/or the compression of material between the drill string and the wall of the wellbore at the site of the packing off.
A stuck drill string may be difficult (or potentially impossible) to remove. The time taken to remove a stuck drill string also results in significant extra costs.
The same problems also apply to other boreholes and the use of other drill string elements.
There is a need, therefore, to provide mechanisms and methods by which stuck drill strings can be more readily removed from boreholes.
Accordingly, the present invention seeks to ameliorate one or more problems associated with the prior art.
The foregoing discussion is intended only to illustrate the present field and should not be taken as a disavowal of claim scope.